# NCERT Solutions for Class 9 Science Unit 11

## Work and Energy Class 9

### Unit 11 Work and Energy Exercise Solutions

Work is a very normal term, usually one we dislike; "clean up your room !", "mow the lawn !" etc.
This word of "work" brings to mind pushing, pulling, walking back and forth. The very unusual thing about the day-to-day usage is that it is almost identical to the Physics usage of the term.
"The WORK done on an object, is the product of the average force on it and the distance travelled in the direction of the force."
"ENERGY is the ABILITY of an object to do work for whatever reason."

### Question : Solutions of Questions on Page Number : 148

Q1 :

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case? When a force Facts on an object to displace it through a distance Sin its direction, then the work done Won the body by the force is given by:

Work done = Force ÃƒÆ’Ã¢â‚¬”Displacement

W= F ÃƒÆ’Ã¢â‚¬” S

Where,

F= 7 N

S= 8 m

Therefore, work done, W= 7 ÃƒÆ’Ã¢â‚¬”8

= 56 Nm

= 56 J

Q2 :

When do we say that work is done?

Work is done whenever the given conditions are satisfied:

(i) A force acts on the body.

(ii) There is a displacement of the body caused by the applied force along the direction of the applied force.

Q3 :

When do we say that work is done?

Q4 :

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Q5 :

Define 1 J of work.

Q6 :

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Q7 :

What is the kinetic energy of an object?

Q8 :

Write an expression for the kinetic energy of an object.

Q9 :

The kinetic energy of an object of mass, mmoving with a velocity of 5 m s - '1is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Q10 :

What is power?

Q11 :

Define 1 watt of power:

Q12 :

A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Q13 :

Define average power.

### Exercise : Solutions of Questions on Page Number : 158

Q1 :

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

”¢Suma is swimming in a pond.

”¢A donkey is carrying a load on its back.

”¢A wind mill is lifting water from a well.

”¢A green plant is carrying out photosynthesis.

”¢An engine is pulling a train.

”¢Food grains are getting dried in the sun.

”¢A sailboat is moving due to wind energy.

Q2 :

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Q3 :

A battery lights a bulb. Describe the energy changes involved in the process.

Q4 :

Certain force acting on a 20 kg mass changes its velocity from 5 m s - '1to 2 m s - '1. Calculate the work done by the force.

Q5 :

A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Q6 :

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Q7 :

What are the various energy transformations that occur when you are riding a bicycle?

Q8 :

Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Q9 :

A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Q10 :

A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Q11 :

What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

Q12 :

Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

Q13 :

A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Q14 :

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Q15 :

Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

Q16 :

An object of mass, mis moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?

Q17 :

Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

Q18 :

In each of the following a force, Fis acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. Q19 :

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Q20 :

Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.

Q21 :

A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?